Optical information recording/reproduction apparatuses using disk-shaped optical information recording media are available in a variety of different forms, including compact disk (CD) players and mini-disk (MD) recorders. Advances in recent years have led to increasingly small information recording media and recording/reproduction apparatuses.
Hereinafter, an example of an above-mentioned conventional optical information recording medium and optical information recording/reproduction apparatus will be described with reference to the drawings.
FIG. 11 is a cross-sectional diagram showing the configuration of a conventional optical information recording medium and an optical information recording/reproduction apparatus. In FIG. 11, a substrate 51 has a light incidence surface 51c and an information layer 51e parallel to the light incidence surface 51c. The substrate 51 is provided with a turntable set surface 51b, which is parallel to the light incidence surface 51c, formed near its center and a centering portion 51a, which is a center aperture. A magnetic clamping plate 52 made of a magnetic material is fastened onto the surface of the substrate 51 opposite the turntable set surface 51b, forming an optical information recording medium. The reference numeral 53 denotes a turntable on which the substrate 51 is placed, and the turntable 53 is provided in a single unit with a turntable centering member 53a. A magnet 54 is fastened to the middle of the turntable 53, forming a support member. The turntable 53 is fastened to a spindle motor 55, which is a rotating member. The turntable centering member 53a and the central rotation axis of the spindle motor 54 coincide with one another. A pickup 56 for irradiating light onto the light incidence surface 51c of the substrate 51 so as to store and reproduce information is disposed on the same side as the turntable 53 with respect to the substrate 51.
Hereinafter, the operations of the optical information recording medium and the optical information recording/reproduction apparatus configured as above are described below.
The substrate 51 is placed on the turntable 53 so that the centering portion 51a provided in the substrate 51 engages with the turntable centering member 53a in the turntable 53. At this time, the light incidence surface 51c of the substrate 51 is on the side of the turntable. The substrate 51 is pushed against the turntable 53 due to the attractive force that is generated between the magnet 54 and the magnetic clamping plate 52. The turntable 53 and the substrate 51 rotate as a single unit due to the rotation of the spindle motor 55. The light emitted from the pickup 56, which is on the same side as the turntable 53 with respect to the substrate 51, is irradiated onto the information layer 51e from the light incidence surface 51c of the substrate 51, storing or reproducing information. As the pickup 56 moves in the radial direction of the substrate 51, the light emitted from the pickup 56 can be irradiated over the entire surface of an information region 51d, storing or reproducing information.
In recent years, disk-shaped information recording media such as optical disks have become progressively more compact, leading to smaller outer shapes and smaller areas onto which information can be stored. With the conventional optical information recording/reproduction apparatus in FIG. 11, the pickup 56 cannot be moved more inward than the position of the radius R2 due to interference by the spindle motor, and information cannot be stored or reproduced in the region near the center of the substrate 51 because the light emitted from the pickup cannot be irradiated there. Also, the turntable set surface 51b, which comes into contact with the turntable 53, of the substrate 51 is a region in which information cannot be stored or reproduced because the light emitted from the pickup 56 cannot be irradiated there. For example, with a mini-disk, the outer diameter D51 of the substrate 51 in FIG. 11 is 64 mm and the diameter of the information region is from 32 mm to 61 mm. The percentage of the area of the information region with respect to the total area of the 64 mm diameter is 66%. When the diameter of the substrate is set to a small diameter of 50 mm, then the percent thereof that is the information region becomes 47%. In order to increase the percentage of the information region, it is necessary to reduce the outer diameters of the turntable 53 and the turntable set surface 51b so as to reduce the region in which recording and reproduction is not possible. However, when the outer diameters of the turntable 53 and the turntable set surface 51b are reduced, the support for the substrate 51 becomes unstable, leading to the risk that the rotation of the spindle motor 55 may not be transmitted accurately or that the increase in surface vibration of the substrate 51 may make the recording or reproduction of information with respect to the substrate 51 unstable. Thus, it was not possible to significantly reduce the outer diameter of the turntable 53, and this led to the problem that the percentage of the information region with respect to the total area of the substrate 51 becomes smaller when the outer diameter of the substrate 51 is reduced.